This invention relates to an improvement in bottom-contacting fish nets in which frictional resistance encountered between the net and the substrate is significantly reduced. This results in a reduced hauling energy requirement.
Man has used seines for capturing fish or other aquatic animals from times long before the advent of recorded history. By definition, a seine is a long net which typically exceeds in top-to-bottom height the depth of body of water in which it is used. The bottom of the siene is weighted to keep it in contact with the substrate. The upper part of the siene is normally maintained on or near the surface by floats or other buoyant means. Seines are of various types, but they all operate in a fashion in which they enclose or trap the aquatic life being harvested. The entrapment is often accomplished by towing one end of the seine around the circumference of a given area until it meets the opposite end. The enclosure can then be drawn onto land to recover the enclosed creatures. Alternatively, the diameter can be made progressively smaller so that the entrapped creatures are concentrated and can be recovered by dip nets or other means. Seines are also used to recover fish or other aquatic animals which have been farmed in ponds or raceways. The latter are long rectangular tanks which are rarely more than two meters in depth. In harvesting fish from a raceway, the seine is placed transversely at one end and drawn lengthwise down the enclosure. The entrapped aquatic creatures are then recovered when the pass is completed at the far end of the raceway.
Another device which may be used for capturing aquatic creatures is the trawl. The trawl is a wide, funnel-shaped net in which the mouth is held open by a framework, floats, paravanes or some other means. The trawl can be drawn through the water at any desired depth. Frequently, it too is run in contact with the bottom.
One of the problems with both seines and bottom-operated trawls is the frictional force between the device and the bottom substrate. The frictional effect is exacerbated by the fact that the leadline, which holds the device in contact with the bottom, tends to dig into and then plow through, the substrate. The leadline typically is a cord or line of adequate size and strength which carries cylindrical or ellipsoidal lead weights at regular intervals. It is also frequently found in the form of a steel chain, especially on trawls. The term "leadline" will be used to designate this weighted member regardless of its physical form.
The amount of friction between the net and the bottom has a major effect on the pulling force required to move the device through the body of water. Bottom friction is determined primarily by the physical shape or configuration of the seine or trawl which is in contact with the substrate, and by the nature of the substrate itself. The leadline of a seine or trawl will tend to dig into a muddy or silty substrate far more than it will a hard substrate. In some extreme cases, the device can become stuck or "mudded in" the substrate material to the point that no further forward movement is possible. It is not unknown in these circumstances that an expensive trawl must be abandoned.
Surprisingly, the art does not appear to have broadly addressed the problem of reducing friction with the bottom. U.S. Pat. No. 3,058,248 discloses the use of a rubber-cased rolling bobbin which is designed to keep the ground line of the trawl net close to, but somewhat off the bottom. U.S. Pat. No. 3,231,998 also shows a rolling device to keep the ground line of a trawl free of the substrate. The latter patent also teaches the use of a resilient tickler line which rides on the bottom ahead of the mouth of the trawl in order to startle bottom-dwelling fish so that they swim up into the net.